i'm running 3gb on my windows XP 32-bit right now, and honestly i see no difference in performance to 1gb until i finally did a stress test.
I opened Quake 4, Oblivion, Guild wars, Rome: total war, aim, 500mb worth of instances of firefox, and started playing music on winamp and itunes.
I played each game......any guess what happened? I was using 2.6gb of Paging files. i have a 7800GT and a AMD 4400+ X2 for the BTW....and no OCing.
i played oblivion, lowered the setting from ultra high to high and got no lag.
I played quake 4, and lowered the settings to medium, no lag.
I played guild wars, lagged until i got it to medium low settings.
Rome: total war i played at full graphics.
My CPU was at 100% constantly, and the 3gb did its job.....just dont ask how i didnt get any lag....i'm trying to figure that out myself.
EDIT: my point is that 4gb is overkill for any XP, but if i was running vista it might have slowed down a little, either way if your computer is lagging at 2gb in a game, something is wrong. Try reinstalling or something.
EDIT2: just for the **** of it i tried it with 1gb, i had to restart my computer in less than 2 minutes....
Computer noob in training.